1. Field of the Invention
This invention relates generally to object transport apparatus, and relates more particularly to a step-dwell transport apparatus and method for use in an in-line sputtering system.
2. Description of the Relevant Art
Sputtering is a process that is used for depositing material coatings onto objects. Sputtering may be used, for example, to apply optical coatings, to deposit metallization layers on semiconductors, to coat chrome masks, to coat flat panel displays, and to apply magnetic coatings to memory disks. The sputtering action can be reversed to etch away material from the surface of objects. Sputtering operations are typically carried out in an argon environment at reduced pressure, which necessitates the use of a process chamber sealed from the atmosphere and the use of air locks or other provisions for loading objects into and unloading objects from the process chamber. To obtain high quality coatings, it is essential that particulates and other contaminants be eliminated from the process chamber.
In the prior art, there are two general types of sputtering systems used for sputtering magnetic coatings onto memory disks: (1) those using a circular sputtering source with the disk or disks being held stationary during sputtering, and (2) those that continuously transport a disk or disks past a stationary strip source.
The concept of sputtering stationary disks is utilized in a disk sputtering system built by Varian of Santa Clara, Calif. Varian's MDP 1000 provides several separate process chambers for sputtering operations and a walking beam mechanism for transporting the disks through the system. The walking beam mechanism operates in conjunction with pedestals that are associated with each process chamber. The walking beam picks up an entire line of disks and advances the disks one station forward to the next pedestal. After such an advance, each pedestal raises its disk into the process chamber and seals the process chamber. When the process operation is complete, the pedestal is lowered and the walking beam mechanism transports the disk to the next pedestal. One significant drawback to such a complex transport mechanism is that particulates are more likely to be generated than with a simpler mechanism. Another drawback is that the system is difficult to expand to accommodate additional processing stations due to the complexity of the transport mechanism. Further, any disk that happens to fall off a pedestal or the walking beam is likely to jam the mechanism.
A sputtering system built by Gartek of Sunnyvale, Calif. also incorporated the concept of sputtering to stationary disks. The Gartek system included a U-shaped channel extending through an in-line process chamber containing several processing stations. Disks were conveyed through the process chamber by a queue of abutting carriers that slid in the U-shaped channel. Each carrier included a square rod that fit into the U-shaped channel, and included arms extending upward that retained a disk. Five carriers were loaded into an entrance chamber at the start of a batch run. After the entrance chamber was pumped down, a carrier was pushed into the process chamber, which pushed other carriers ahead to the next processing stations and pushed the leading carrier into an exit chamber. While the carriers were stationary, the processing operations were performed. After the five carriers had been transferred into the process chamber, two gate valves closed to isolate the process chamber from the entrance and exit chambers. The entrance and exit chambers were then vented to atmosphere, and processed disks were unloaded from carriers in the exit chamber and more unprocessed disks were placed on carriers and loaded into the entrance chamber. The batch mode operation was one significant drawback of the Gartek system, due to the time required to load and unload the entrance and exit chambers, and also due to inconsistent sputtering performance caused by the interruptions of loading and unloading. Another significant drawback was the tendency of the U-shaped channel to collect sputtered particles, which were flaked off by the carriers and contaminated the process chamber, and which caused the carriers to seize in the channel.
Several presently available sputtering systems continuously transport disks past a stationary sputtering source, including those made by Circuits Processing Apparatus of Fremont, Calif., Leybold-Heraeus of the Federal Republic of Germany, and TorrVac of Simi Valley, Calif. These systems orient the disks vertically on pallets, which are separately conveyed through a process chamber containing one or more in-line processing stations. Material is sputtered onto the disks as the pallets are continuously moved past stationary sputtering sources. Some such sputtering systems operate on a batch mode basis, wherein each process cycle consists of loading several pallets into an entrance air lock, processing each pallet and storing processed pallets in an exit air lock, and then unloading the exit air lock after all of the pallets have been processed. Since operation in batch mode involves a substantial amount of down time while the air locks are loaded and unloaded, other sputtering systems provide for pass through air locks that automatically load and unload single pallets into and from the process chamber, plus an automatic pallet return system that returns pallets from the exit side of the system to the entrance side. The transport mechanisms utilized in continuous type sputtering systems are typically chain or gear driven conveyors that tend to generate particulates within the process chamber. Another source of particulates are the top guide rollers that are typically used to guide and support the upper portions of the pallets.
Aside from particulate generation by the transport mechanisms, continuous sputtering systems have a more fundamental problem, that of magnetic layer modulation. A memory disk with a sputtered magnetic layer is used in a disk drive to magnetically store digital data in concentric tracks. If the strength of a data signal recorded on a track is modulated, i.e., varies cyclically as the disk is rotated, then the data content of the data signal is much more difficult to detect accurately. Magnetic layers applied by a continuous sputtering process tend to modulate the signal strength of data signals due to both variations in the layer thickness and skewed magnetic particle orientation. If the strip source used for sputtering the magnetic layer has any irregularities across its length that cause increased or decreased deposition rates, then a chord of increased or decreased layer thickness will be formed on the disk. When the disk is rotated, this chord will pass under the magnetic pick-up twice during each revolution, thereby modulating the magnetic signal. Additionally, magnetic particle orientation contributes to signal modulation. The orientation of the magnetic particles is radially skewed because particles sputtered by the strip source first contact the surface of the disk before the surface is normal to the strip source, which orients the magnetic particles toward the direction of pallet travel. When such a disk is rotated, the orientation of the magnetic particles continuously changes with respect to the magnetic pick-up, and thus contributes to signal modulation.